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Lecture 4. Detectors for GC.
Thermal conductivity detector (TCD).
% TCD detector consists of an electrically-heated wire or thermistor &tungsten-rhenium wire'. The temperature of the sensing element depends on the thermalconductivity of the gas flowing around it. Changes in thermal conductivity#
such as when organic molecules displace some of the carrier gas# cause atemperature rise in the element which is sensed as a change in resistance.
Response is universal and proportional to concentration.
,est gases for TCD: or e# because of highest thermal conductivity &/.+0/and /.+1+ 23&45m5s'# respectively# for (/./1'. or helium and hydrogen thetemperature conductivity lowers when solute is eluted.
The sensitivity is inversely proportional to flow rate: The detector is moresensitive at lower flow rates.
Detection limit: 1// pg3ml.6inear response range: 7 +/8.
To increase sensitivity the temperature of the bloc9 should be maintained atlowest temperature that allows all the solutes remain gaseous.
ilament
Gasflow
outGasflowin
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Lecture 4. Detectors for GC.
Flame-Ionization Detector! (FID).
The eluate is burned in a mi"ture of hydrogen and air. Carbon atoms &e"cept-C and -C' produce C radical# which are thought to produce C
